In standard telephony, voice signals exchanged between parties modulate an electrical signal in base band which is transmitted by two-wire lines, often twisted pairs. Coding and modulation processes have enabled the same transmission principles to be used to transmit digital data. In this case, the digital data to be transmitted is modulated at the transmitter and demodulated at the receiver by modems in order to be transmitted via the same paths as the voice signal. Consequently, these signals modulated in this way will be referred to as signals modulated in a natural analogue manner. Natural analogue signals are distinguished from digital signals. Digital signals, typically binary signals for modulation in base band, require prior coding.
Also, to send voice signals via a digital channel, vocoders convert natural analogue signals into streams of bits timed at a standardized rate. Such conversions are encountered in particular in mobile telephony, in which all processing is digital, including modulation and radio transmission.
In the context of the Internet, the signals exchanged between terminals and Internet sites visited by users of the terminals are digital signals formatted in accordance with a particular protocol which is specific to the Internet and is known as the Internet Protocol (IP). An Internet connection terminal therefore includes appropriate converter circuits.
Initially, Internet connections entailed only visiting sites, in accordance with the following principles. Using an Internet terminal, a user enters a site address. This process is often greatly simplified and amounts to no more than double-clicking the mouse on an icon on the screen of the terminal. The microprocessor of the terminal then finds the address of the site corresponding to the icon in a memory of the terminal, constructs a message conforming to the IP using that address, and transmits the message to circuits of a service provider via a modem.
The service provider circuits transmit a request to the site designated by the address message. The designated site receives the request and sends a file representing a page to the terminal. Sending to the terminal is relatively easy because, in the IP format, the address of the user terminal is transmitted at the same time as the address designating the target site.
This basic, but already particularly interesting, facility has now had telephone call services added to it. The document WO 97/47118 describes a system for telephone communication during an Internet session: the user terminals are personal computers, each of which has a loudspeaker and a microphone connected to a sound card. The sound card includes:                a digitizing and compression system for converting voice signals from the microphone into digital signals, and        a digital-to-analogue converter and decompression system for converting digital voice signals into analogue signals that can be fed to the loudspeaker.        
The processor carries out a formatting operation to construct IP packets from the digital signals from the sound card. A modem transmits the digitized voice signals formatted to the IP format over a standard analogue telephone line. Outside Internet sessions, the telephone line can be used by a standard telephone. However, the telephone cannot be used during an Internet session because the line is busied by the modem.
To start an Internet session, the terminal sets up a call to the circuits of an Internet service provider, which allocate an Internet address to the terminal.
In the event of an outgoing telephone call from the Internet terminal, a telephony application is executed in the terminal. The user enters the number of the called party. The IP packets are received by the circuits of the Internet service provider and are then forwarded to a telephony server which sets up a telephone call to the called party line:                If the called party is a standard telephone, the call set up between it and the server is a standard telephone call. The server sends decompressed analogue voice signals.        If the called party has a personal computer with a sound card, a microphone and a loudspeaker, and is running a telephony application using the same server, the server alerts the called party by means of an IP packet. If the called party decides to take the call, the server sends them the voice signals in compressed digital form in IP packets.        
In the case of an outgoing telephone call from a standard telephone to a called party using an Internet terminal, if the Internet terminal is in use and a telephony application is running on it, the calling party enters the number corresponding to the line to which the Internet terminal is connected. However, the telephone network forwards the call to the telephony server, which knows that the called telephone number corresponds to an IP address for setting up a telephone connection using the Internet protocol. It sends the called party an alert message in the form of an IP packet. If the called party decides to take the call, the server digitizes the voice signals from the calling party, compresses them, and assembles them into IP packets. The IP packets are sent to the Internet service provider and are then forwarded to the Internet terminal of the called party.
Compared to the standard methods, the above method of routing telephone calls has the advantage of using a single infrastructure, that of the Internet, to transmit data and voice at the same time.
However, the method has the disadvantage that the Internet terminal must be fitted with an additional circuit card for digitizing and compressing voice signals, and for the converse operations, and enabling connection of a loudspeaker and a microphone or a telephone. If the Internet connection terminal is an IBM-compatible personal computer, it can be fitted with a sound card enabling it to perform digitization and compression and to enable the connection of a telephone or a loudspeaker and a microphone. This represents an additional cost. Also, installing a sound card is not possible in some Internet terminals.